Heretofore, there have been known fuel cells for use in a stack which utilize an aqueous solution of electrolyte. In such a fuel cell, a unit cell comprises a porous matrix holding the aqueous solution of electrolyte and two porous layers forming each either an anode or a cathode. The porous layers carry catalysts which form two catalyst layers to be in contact with the matrix. Reactant gases diffuse through the porous layers and react electrochemically in the catalyst layers: thus, a three phase gas-catalyst (solid)-electrolyte (liquid) reaction may occur.
These unit cells are separated from one another by a layer of gas impermeable compact material, so that a fuel gas and an oxidizing gas utilized as reactant gases may not be mixed. The constituent elements of such a unit cell are made of corrosion resistant materials, such as carbonaceous materials and corrosion resistant alloys, which can endure the severe operational environment of the fuel cell, for example, acids, alkalies, relatively high temperatures or the like, and which are good conductors of electricity and heat. A fuel cell may be made by stacking several tens to hundreds of such unit cells so that a predetermined voltage and current may be obtained.
Recently, the development of fuel cells and related systems thereof have been demanded for a generator of clean energy or for a freely openable and closable generator to be utilized in the leveling of operations of thermoelectric or hydroelectric power plants or the saving of resources by improving the efficiency of energy.
Substrates of a fuel cell in a stack may be classified into two groups, namely, monopolar and bipolar, depending on the nature and kinds of gas impermeable layers for preventing the mixing of reactant gases and of porous layers as gas diffusion layers.
Bipolar-type electrode substrates comprise a gas impermeable layer and two gas diffusion layers, both of which are integrated with said impermeable layer by adhesion or coupling. Therefore, the thickness of a stack comprising numbers of unit cells can be smaller and, additionally, both electric and thermal contact resistances between the layers can be significantly reduced. Moreover, the mechanical strengths of the stack as well as of the electrode substrate may be markedly higher. Thus, bipolar electrode substrates may be more advantageous to an improvement of the performance of a fuel cell and to compactness of a device, as compared with monopolar electrode substrates.
Bipolar electrode substrates for fuel cells comprising a separator provided with channels for reactant gases are known, wherein the channels are made by ribbing both surfaces of an gas impermeable carbonaceous thin plate.
The present inventors have provided an electrode substrate based on short carbon fibers and having excellent properties, this substrate being provided with channels for reactant gases near the center of a porous carbonaceous layer as a gas diffusion layer. The substrate may be prepared by press molding and heat treatment which are easier than mechanical processes such as ribbing and boring. See Japanese Patent Application Laying Open No. 68170/84.
In the course of preparation of such an electrode substrate, a porous carbonaceous layer having desirable open pores can be obtained by utilizing short carbon fibers as a base material, a binder of thermosetting resin having a specific carbonizing yield, and a pore regulator having a specified particle size and decomposing thermally at a temperature higher than the molding temperature. However, during the calcination process of a shaped article, the exfoliation of a porous carbonaceous layer from a gas impermeable layer (a dense carbonaceous layer) may inevitably take place, and in particular, such exfoliation may result in a low yield in the manufacture of large substrates in spite of an improved planning of the temperature-increasing procedure. Thus, a further improvement of preparation of electrode substrates has been required.